Download AC voltage controller

AC to AC Converters
Outline
AC voltage controllers
Other AC controllers
Thyristor cycloconverters
Matrix converters
Single-phase AC voltage controller
VT 1
VT2
u1
u1
io
O
uo
t
R uo
O
t
io
The phase shift range
(operation range of phase
delay angle):
O
u VT
t
0 ≤α≤ π
O
t
Inductive (Inductor- resistor) load , operation principle
u1
VT 1
O
io
uG1
VT2
R
u1
t
uo
L
O
uG2
O
uo
t
O
io
t
O
t
t
The phase shift range:
φ ≤α ≤ π
uVT
O
t
4.1.2 Three-phase AC voltage controller
Classification of three- phase circuits
ia VT1
Ua0'
a
u a VT
3
n
ub
ia
VT4
b
VT5
uc
ua
n'
VT6
c
n
Line- controlled Δ connection
ia
ua
a
a
ia
ub
b
b
n
ub
uc
ub
uc
Y connection
n
b
c
VT2
ua
a
uc
c
Branch-controlled Δ connection
c
Neutral-point controlled Δconnection

3- phase 3- wire Y connection AC voltage controller
i a VT 1
ua
VT3
n
ub
uc
VT5
Ua0'
a
VT4
b
n'
VT6
c
VT 2
For a time instant, there are 2 possible conduction states:
–Each phase has a thyristor conducting. Load voltages are the same as the source
voltages.
–There are only 2 thyristors conducting, each from a phase. The load voltages of
the two conducting phases are half of the corresponding line to line voltage,
while the load voltage of the other phase is 0.
Other AC controllers
Integral cycle control—AC power controller
VT1
VT2
u1
io
Conduction
uo angle =
2 U1
uo
R O 
M
2
M
2N
M
u1
uo,io
3
M
4
M
t
Line period
Control period = M *Line period
= 2
Circuit topologies are the same as AC voltage controllers.
Only the control method is different.
Load voltage and current are both sinusoidal when thyristors are conducting.
Electronic AC switch
Circuit topologies are the same as AC voltage controllers. But the back- to- back
thyristors are just used like a switch to turn the equipment on or off.
Application—Thyristor-switched capacitor (TSC)
I
U

TSC waveforms when the capacitor is switched in/out
uVT 1
uC
iC
C
VT2
t
uC
VT1
us
us
t
uVT 1
t
VT1
iC
t
VT2
t1
t2
The voltage across the thyristor must be nearly zero when switching in the
capacitor, and the current of the thyristor must be zero when switching
out the capacitor.

TSC with the electronic switch realized by a thyristor and an antiparallel diode
uVT
uC
iC
VT
us
C
VD
us
t
uC
t
uVT
t
VD
iC
VT
t
t 1 t2 t 3 t 4
The capacitor voltage will be always charged up to the peak of source
voltage.
The response to switching- out command could be a little slower
(maximum delay is one line-cycle).
Chopping control—AC chopper
AC chopper
Modes of operation

VD 1
V1
i1
u1
V2
VD 2
V3
VD 3
VD 4
uo
V4
u>0, io >0: V1 charging,
V3 freewheeling
图4-7
u>0, io <0: V4 charging, V2 freewheeling
u<0, io >0: V3 charging, V1 freewheeling
u<0, io <0: V2 charging, V4 freewheeling
R
L
Thyristor cycloconverters
Single- phase thyristor-cycloconverter

Circuit configuration and operation principle
P
N
Z
uo

ap=
π
2
Output
voltage
Average
ap=0
output voltage
ap =
π
2
t

Single- phase thyristor-cycloconverter
Modes of operation
uo
u o,io
O t1
iP
uP
uo
t2
t4
t3
t5
t
O
iN
uN
t
uo
uP
io
io
uN
t
uo
O
iP
P
N
O
iN
t
O
t
Rectifi
Inver
cation
sion
blocking
blocking
Rectifi
Inver
cation
sion
Typical waveforms
uo
t
O
io
O
t
1
3
2
4
6
5
Three- phase thyristor-cyclo converter

The configuration with common input line
图4-24

The configuration with star-connected output
Typical waveforms
Output voltage
200 t/ms
Input current with
Single-phase output
Input current with
3-phase output
200 t/ms
200 t/ms
Input and output characteristics
The maximum output frequency and the harmonics in the output voltage
are the same as in single-phase circuit. Input power factor is a little
higher than single-phase circuit. Harmonics in the input current is a
little lower thanthe single- phase circuit due to the cancellation of some
harmonics among the 3 phases.
To improve the input power factor:
–Use DC bias or 3k order component bias on each of the 3 output
phase voltages

Features and applications
Features:
–Direct frequency conversion—high efficiency
–Bidirectional energy flow, easy to realize 4- quadrant operation
–Very complicated—too many power semiconductor devices
–Low output frequency
–Low input power factor and bad input current waveform
Applications:
–High power low speed AC motor drive

Matrix converter

Circuit configuration
input
b
a
c
u
S1
S1
S1
1
2
3
S2
S2
S2
1
2
3
S ij
v
output
w
S3
S3
S3
1
2
3
a)
b)

Usable input voltage

Features
Direct frequency conversion—high efficiency can realize good input and
output waveforms, low harmonics, and nearly unity displacement
factor
Bidirectional energy flow, easy to realize 4- quadrant operation
Output frequency is not limited by input frequency
No need for bulk capacitor (as compared to indirect frequency converter)
Very complicated—too many power semiconductor devices
Output voltage magnitude is a little lower as compared to indirect
frequency converter.